The invention relates to a rotor and a method for welding an element of a rotor. Particularly, but not exclusively, the invention relates to a low alloy, high temperature rotor for use in a turbine and a welding method, which method can be used in assembling components of the rotor together during manufacture or for repairing the rotor post-manufacture.
Turbine rotors are typically constructed from low alloy steel. In one known construction, the rotor comprises a monobloc body formed from a single forging. In another known construction, the rotor body comprises a series of individual disks, which are welded together. In each case, blades are disposed upon the periphery of the body by a suitable retaining mechanism, of which there are various types. The invention is particularly applicable to these types of rotor, but other types, of which there are many, are not excluded.
Any metal article subjected to stress over a period of time will experience what is known as xe2x80x9ccreepxe2x80x9d. Creep is the plastic deformation of the metal over that period and is therefore time-dependent strain. Creep is extremely sensitive to temperature, and sensitivity increases rapidly with rising temperature. After a period of time, creep will ultimately conclude by the generation of a fracture.
Whilst the severe consequences of a fracture should not be underestimated, the situation is mitigated by the fact that it is possible to predict the progression of creep within a rotor with a good degree of accuracy, providing the operating conditions for the rotor are known and controlled. The consequence is that a xe2x80x9ccreep-lifexe2x80x9d for a rotor can be predicted, therefore enabling the period of time before a failure should occur to be established. This means that the useful working-life of the rotor (the xe2x80x98service lifexe2x80x99) can be determined in advance, thereby ensuring that it is decommissioned or repaired before a failure occurs.
It is desirable for the creep-life of both the initially manufactured rotor and the repaired rotor to be as long as possible. Furthermore, it is highly desirable that the creep-life can be predicted with a high degree of accuracy, not only to ensure that a fracture or failure is avoided, but also to ensure that the service-life (which will always be shorter than the creep-life, the difference being a safety margin) will be relatively close to the creep-life of a particular rotor, thereby avoiding any unnecessary redundancy of use.
A rotor that is approaching the end of its service life is normally repaired by a welding process. Although this invention is highly applicable to a welding process used in the initial manufacture of a rotor, it has particular applicability to a repair process and further discussion will therefore concentrate on a repair process.
The post-repair rotor will have its creep strength affected not only by the parent metal (from which the rotor was originally manufactured), but also the metal used in the production of the repair weld. It is therefore necessary to select a weld metal that will provide sufficiently adequate creep characteristics following the repair. It is, however, also necessary to take into account other characteristics of the post-repair rotor and this will significantly include the thermal expansion coefficients of the various integers of the rotor. If the co-efficient of thermal expansion of the weld metal is significantly different from that of the parent metal, distortions and additional operating stresses will occur, both of which would not only affect the further service-life of the rotor, but also complicate the prediction of the creep-life, requiring, at least, a greater error margin to be built into the measurements and predictions that would need to be made.
With the foregoing requirements in mind, a number of weld metal types have, for example, been used for a typical rotor type, manufactured from 1% CrMoV low alloy steel.
A first such weld metal type has a creep strength which is at least as great as that of the parent metal, but is a higher alloy material, with a lower co-efficient of thermal expansion; one example of this is 12% CrMoV used in association with a 5% Cr weld metal layer between the parent metal and the 12% CrMoV weld metal. A further known alloy material has physical properties which are similar to that of the original parent rotor material, but with lower creep strength, for example low carbon 1% CrMoV or 2% CrMo.
The first of these types, whilst having a sufficiently adequate creep strength, introduces uncertainty by virtue of the thermal effects of having a lower co-efficient of thermal expansion relative to the rotor steel. The latter suffers from having a lower creep strength than the rotor steel, so that the prolongation of the rotor life would be limited.
It is therefore an object of the invention to provide a process which results in a rotor that does not have the creep life normally attributable to the parent metal substantially shortened as a consequence of the creep-life of the weld metal and in which the co-efficient of thermal expansion of the parent metal and weld metal are comparably similar or identical.
In an exemplary embodiment, a method of forming a rotor comprises the steps of providing a rotor element formed from steel and welding the rotor element, using a welding process employing a weld metal which comprises: from 0.04 to 0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese, from 0 to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6% chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from 0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03% nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium.
In a further exemplary embodiment, a method of forming a rotor comprises removing at least a portion of a creep-life expired region of a first rotor element formed from a steel, replacing the removed portion of the first rotor element by welding the rotor element with a weld metal or by welding a second rotor element to the first rotor element with the weld metal, heat treating the rotor at a temperature range of 650xc2x0 C. to 750xc2x0 C., and machining the rotor to remove at least a portion of the weld metal. The weld metal comprises 0.04 to 0.1% carbon, 0 to 0.5% silicon, 0.1 to 0.6% manganese, 0 to 0.01% sulphur, 0 to 0.03% phosphorous, 1.9 to 2.6% chromium, 0.05 to 0.3% molybdenum, 0.2 to 0.3% vanadium, 0.02 to 0.08% niobium, 1.45 to 2.1% tungsten, 0 to 0.03% nitrogen, 0.0005 to 0.006% boron, and 0 to 0.03% aluminium.
An exemplary rotor for a turbine comprises a rotor element and weld metal welded to the rotor element. The weld metal comprises: from 0.04 to 0.1% carbon, from 0 to 0.5% silicon, from 0.1 to 0.6% manganese, from 0 to 0.01% sulphur, from 0 to 0.03% phosphorous, from 1.9 to 2.6% chromium, from 0.05 to 0.3% molybdenum, from 0.2 to 0.3% vanadium, from 0.02 to 0.08% niobium, from 1.45 to 2.1% tungsten, from 0 to 0.03% nitrogen, from 0.0005 to 0.006% boron and from 0 to 0.03% aluminium.